1. Field of the Invention
This invention relates to drill bits used to create boreholes through subterranean formations and, more particularly, to apparatus and method used for stabilizing a drill bit and drill string during subterranean drilling.
2. Setting of the Invention
In the exploration and production of hydrocarbons, a rotating drill bit is used to create a borehole through the Earth's subterranean formations. Users of the drill bits and drill bit manufacturers have found that increased penetration can be achieved by more precisely controlling the weight on bit (WOB) and increasing the rotational speed (RPM) of the drill bit. However, as the RPM has been increased, the drill bit effective life has decreased dramatically because the cutting elements on the drill bit are cracked and occasionally are torn from the drill bit body at the higher RPM's.
Numerous studies have been made to find out what causes such destruction to the cutting elements. The inventors hereof have previously found that a substantial portion of the destructive forces are created by a phenomenon known as "whirl". More specifically, radially-directed, centrifugal imbalance forces exist to some degree on every rotating drill bit and drill string. These forces are created by radial mass imbalances, i.e., mass imbalances across the longitudinal or axial center of the drill bit and drill string, as well as by the dynamic drilling forces which act on the drill bit and drill string. These forces tend to push the drill string and drill bit towards the side of the borehole.
Typically, a drill bit has cutting elements ("cutters") known as gauge cutters which are designed to cut the edge or diameter of the borehole. The centrifugal imbalance forces increase the friction between the cutters contacting the wall of the borehole and the drill bit begins to roll around the wall of the borehole in the opposite direction of the rotation of the drill bit, i.e., normally the drill bit is rotated clockwise and, when the imbalance forces push the cutters into the borehole wall, the drill bit will begin to roll around the borehole in a counterclockwise direction. This phenomenon is commonly called whirl or backwards whirl and is similar to the action of a spirograph.
Once backwards whirl begins, it is self-propagating. The backwards whirling causes the instantaneous center of rotation of the drill bit and drill string to change dynamically as the drill bit backwards whirls around the borehole. The cutting elements travel faster, sideways, and backwards than they do under normal rotation (clockwise) without backwards whirl. Typically, cutters are designed, placed, and supported in a drill bit for cutting while rotating in a predetermined direction, normally clockwise. Since the backwards whirl causes the cutters to contact the borehole in a counterclockwise direction, the forces to which the cutters are exposed are much more likely to damage or destroy the cutters.
Various methods and equipment have been proposed to eliminate or reduce the imbalance forces which initiate backwards whirl, including using dynamically balanced lower drill string assemblies and realigning the cutters to reduce the imbalance forces. As described in U.S. patent application Ser. No. 07/313,126, the inventors of the present invention discovered that backwards whirl can be eliminated and forward whirl induced by creating a low friction bearing zone or pad on the drilling surface of the drill bit. The bearing zone slides on the wall of the borehole and eliminates the friction between the drill bit and the borehole necessary to initiate backwards whirl. The inventors further discovered that a drill bit can be deliberately imbalanced to take advantage of the imbalance forces normally present, i.e., the cutting elements or cutters on the bit can be placed and the mass of the bit and cutters distributed to predetermine the direction of the centrifugal imbalance forces created as the bit rotates and drills. The low friction bearing zone can then be placed so that the predetermined imbalance forces direct and force the bearing zone against the wall of the borehole as the drill bit rotates, thereby preventing backwards whirl by keeping the high friction cutting zone or cutting elements from contacting the borehole wall.
However, even though recent tests conducted by the inventors have shown that the low friction bearing zone can virtually eliminate whirl and greatly increase drill bit life, the dynamics of the drill string itself can cause harmful vibrations as well as contribute to the onset of backwards whirl. Drill string dynamics can, under many circumstances, be so violent as to mitigate the benefits of the low friction bearing zone alone.
A basic concept of the imbalance compensated drill bit disclosed by the inventors in U.S. patent application Ser. No. 07/313,126 is to purposely induce forward whirl in the drill bit so that the drill bit's instantaneous center of rotation or rotational axis does not change with time. If the drill string dynamic forces transferred to the drill bit are sufficient to overcome the force keeping the low friction bearing zone of the drill bit in contact with the borehole wall, the low friction bearing zone will not properly function and backwards whirl may be initiated. The drill bit's cutters will not run true and will be subject to breakage and failure.
Drill collars are commercially available which are intentionally mass imbalanced to induce forward whirl in the drill string. One such drill collar is known as the "wood Pecker" drill collar. These drill collars are sometimes used in attempts to drill straighter boreholes. There is debate within the industry as to the effectiveness of the imbalanced collars for drilling straighter holes, although it is generally agreed that forward whirl is induced by the mass imbalance under normal rotary drilling conditions. There has been no prior disclosure or suggestion of using the imbalanced collars with a drill bit having a low friction bearing zone.
Therefore, there is a need for an apparatus and method which will stabilize both the drill string and drill bit and which will reduce, if not eliminate, backwards whirl of the drill bit and drill string, whether initiated by drill bit imbalance forces or by drill string dynamics.